The invention relates generally to methods and systems for fabricating laser ablation masks and more particularly to approaches to evacuating a vacuum chamber and depositing layers during the fabrication of such masks.
Laser ablation is one available technique for forming features on the surface of a component or forming through holes in the component. Selected portions of the surface of the component are exposed to high energy laser radiation that causes chemical breakdown of the bonds within the exposed material. Localized expansion occurs as a result of the breaking of the chemical bonds. The material which has expanded can be removed using conventional techniques, such as chemical etching.
A laser ablation mask is typically employed to determine the exposure pattern on the surface of the component. The laser ablation mask utilizes a transparent substrate on which one or more layers can be formed and patterned to provide a coating that defines the exposure pattern. The materials for forming the coating are selected to be resistant to damage as a result of exposure to the laser energy. The substrate and the coating should have a resistance to laser-induced damage during ablation operations in which a laser has a strength greater than 150 mJ/cm2. A suitable substrate material is quartz. The coating on the quartz substrate may be a single metal layer, such as a chromium or aluminum layer. Alternatively, the coating may be formed of multiple dielectric layers having alternating high and low refractive indices. U.S. Pat. No. 4,923,772 to Kirch describes a laser ablation mask that is formed of multiple dielectric layers that are patterned to define the exposure pattern.
FIG. 1 is a schematic representation of the use of laser ablation in the process of forming inkjet printheads. The process is described in greater detail in U.S. Pat. No. 5,408,738 to Schantz et al., which is assigned to the assignee of the present invention. A continuous web 10 of polymer material is removed from a roll 12 in a controlled manner. The web material may be the polymer sold by 3M Corporation under the federally registered trademark KAPTON. Sprocket holes 14 along the opposite sides of the web may be used to precisely control movement of the web material relative to a laser source 16, such as an Excimer laser. While not shown in FIG. 1, the laser source is typically located within a laser processing chamber. One or more laser ablation masks 18 can be patterned to define all of the features that are to be formed within the continuous web 10. The features are repeated at a controlled interval, so that duplicate components for an inkjet printhead may be formed from the web, after the web is diced. In FIG. 1, the mask 18 is patterned to define an array of vaporization chambers. In addition to stepping the movement of the continuous web 10, the laser source 16 may be stepped. The step-and-repeat process is continued until a nozzle member is formed. Optics 20 may be used for focusing the laser energy that propagates through the mask 18.
The treated portion of the web then advances to a cleaning station, not shown, where any debris is removed from the web. The next station is a bonding station at which heater substrates 22 are secured to the web at positions conforming to the arrays of vaporization chambers. Each heater substrate may be a silicon die on which resistors are formed in an array that matches the array of vaporization chambers, so that there is a one-to-one correspondence between the arrays. The web can then be cut in order to provide individual printheads 24 that are attached to other components to form inkjet cartridges.
Returning to the laser ablation mask 18 that is used in the ablation station, there are a number of equally important mask-fabrication steps. The material for forming the mask substrate should be selected for its optical properties, since the laser energy propagates through the substrate. Quartz is a preferred substrate material. The substrate should be thoroughly cleaned prior to forming the coating on at least one surface of the substrate. The cleaning process removes trace organic layers, such as remnants of the compounds that are used to polish the quartz substrate. Impurities may strongly influence the lifetime of the laser ablation mask. The coating is then applied to the substrate. Conventional Physical Vapor Deposition (PVD) techniques may be utilized. PVD processing requires that the substrate be placed in a vacuum chamber and that the chamber be evacuated. Often, a mechanical pump is controlled by a roughing valve to reduce the pressure within the chamber to a particular setpoint of pressure. A second pump is then used to provide a high vacuum environment within the chamber.
Materials are introduced into the vacuum chamber to vapor deposit layers. As previously noted, the coating on the substrate may be a single layer of metal or composite metal, or may be a dielectric stack. The dielectric stack includes layers having alternating high and low indices of refraction. Absorption of laser energy by the mask coating is a major cause of degradation of the mask. Therefore, the mask coating should be reflective to light having the wavelength of the laser energy. Reflection from the dielectric stack is a result of the constructive and destructive interference at the interfaces of abutting layers. Each layer preferably has a thickness of approximately one quarter-wavelength of the laser energy to which it will be exposed. Each pair of dielectric layers reflects a percentage of the incident light. By depositing a sufficient number of layer pairs, nearly all of the laser energy is reflected.
The coating can then be patterned using conventional techniques. For example, reactive ion etching (RIE) or ion beam etching (IBE) maybe employed. While the resulting mask may operate well for its intended purpose, the operational life of the ablation mask is limited. Laser-induced damage to ablation masks is still critically dependent upon the level of coating defect density. That is, the damage that occurs as a result of exposure to the high energy laser radiation will increase with increases in defect density. With each failed mask, time must be taken to replace the mask. The equipment downtime required to replace masks reduces production throughput in an inkjet printhead manufacturing process.
What is needed is a method and system for fabricating a high energy radiation mask so as to increase the operational life of the mask.
A method of fabricating a high energy radiation mask includes locating a transparent substrate in a vacuum chamber and then executing at least one of (1) reducing the initial rate of evacuating the chamber relative to conventional evacuation techniques and (2) reducing the deposition rate of silicon oxide layers (e.g., SiO2) in a dielectric stack. When the more controlled evacuation procedure is combined with the slower deposition rate of SiO2, the resulting coating has a surprisingly low defect density. Consequently, the operational life of the mask is extended.
In the first embodiment of the invention, the controlled evacuation of the vacuum chamber includes a two-stage roughing procedure, followed by a high vacuum evacuation step. A first roughing evacuation connection to the vacuum chamber is activated to reduce the pressure to a level below atmospheric pressure. When the chamber environment is reduced to a first threshold pressure (i.e., a first setpoint), a second roughing evacuation connection is activated. The second roughing connection has a maximum purging rate that exceeds the maximum purging rate of the first connection. This may be accomplished by adding a bypass valve to the conventional roughing valve to a pump. The bypass valve may have an orifice that is smaller than the orifice through the roughing valve, thereby providing the difference in the maximum rates of evacuation. In an alternative implementation, the two connections are to separate roughing pumps. This alternative implementation is less preferred, since it requires an additional cost of providing the extra pump.
When the second roughing evacuation connection reduces the chamber environment to a second threshold pressure, the high vacuum connection is activated. The high vacuum connection may be to a diffusion pump or a similar device that is able to achieve and maintain a vacuum pressure required for the material deposition process, such as Physical Vapor Deposition (PVD). Preferably, a dielectric stack is deposited on the substrate. The stack can then be patterned to define a desired exposure pattern. Preferably, the patterning is implemented to form a laser ablation mask for fabricating an inkjet printhead. This requires the substrate coating to include openings for forming an array of vaporization chambers within another substrate.
The different maximum rates of evacuation for the first and second roughing evacuation connections are intended to reduce air turbulence that is created by the vacuum process. For example, in the implementation in which the first connection includes an unconventionally small orifice, while the second connection includes a larger orifice that is conventional to roughing, the initial stage will proceed more slowly (i.e., slower mass removal rate) than is typical. This has two advantages. Firstly, less turbulence will occur so that particulate matter is less likely to settle on the surface of the substrate that is within the vacuum chamber. That is, there is a reduction of the adverse effects of the phenomenon that is sometimes referred to as the Wilson Cloud Effect. Secondly, there is a reduced susceptibility to the process causing water evaporation and condensation, so that liquid is less likely to be introduced onto the surface of the mask substrate. A reduced susceptibility to contamination and water on the surface of the mask substrate reduces the interfacial defect sites between the substrate and the dielectric stack. This also occurs if the dielectric stack is replaced with a single metallic layer, such as chromium or aluminum.
In one implementation of the two-stage roughing process, the activations of the three evacuation connections do not overlap. For example, the first roughing evacuation connection may be activated for the pressure range of 760 Torr to 200 Torr, the second roughing evacuation connection may be activated for the range of 200 Torr to 150 mTorr, and the high vacuum pump connection may be activated for the range that begins at 150 mTorr. However, the preferred implementation includes an overlapping activation of the first and second roughing evacuation connections. This may be achieved by triggering the second connection at 300 Torr, while the first connection does not deactivate until the pressure within the vacuum chamber is reduced to 200 Torr. Because there are fluctuations in vacuum pressure, the overlapping method reduces the likelihood that the first connection will be repeatedly cycled between xe2x80x9cactivatedxe2x80x9d and xe2x80x9cdeactivatedxe2x80x9d states. Thus, the mechanism for switching the connections will be subjected to less wear and tear. The switching mechanism may be one or more relays, but other approaches may be taken without diverging from the invention.
The system for implementing the two-stage roughing process includes the housing for forming the vacuum chamber, a pressure monitoring mechanism which includes pressure setpoint activated relays, first and second roughing valves with bypass channels, and the high vacuum pump. The first and second roughing valves may be connected to a single roughing pump or may be connected to separate pumps. The monitoring mechanism may include a Pirani gauge and the pressure setpoint activated relays, but other devices may be employed.
As previously noted, the second embodiment of the invention includes a step of depositing the silicon oxide (preferably SiO2 that is evaporated from an SiO2 source material) at an unconventionally low rate. The optimal range was determined to be a deposition rate of 1 xc3x85/second to 3 xc3x85/second. A deposition rate above this range defines a process that is susceptible to a high coating defect density. On the other hand, a deposition rate below the range may result in poor adhesion of the dielectric stack to the mask substrate. Since the deposition rate is subject to fluctuations, the most preferred implementation targets the deposition rate of SiO2 at 2.0 xc3x85/second, so that the fluctuations are likely to remain within the range of 1.6 xc3x85/second to 2.4 xc3x85/second. The coating material that is deposited to form the layers between the SiO2 layers is preferably deposited at the same rate as the SiO2, but the quality of the high refractive index layers is typically less sensitive to deposition rate. An acceptable material is hafnium oxide (e.g., HfO2). The alternating pattern of SiO2 and HfO2 provides the desired alternating pattern of low refractive index material and high refractive index material, respectively.
An advantage of the invention is that the reduction in the density of defects within the laser ablation mask reduces laser-induced damage during use of the mask. As a result, the operational life of the ablation mask is extended. It follows that the frequency of substituting worn ablation masks with replacement masks will decrease during the fabrication of consumer products, such as inkjet printheads, thereby increasing production throughput.